C.S.I. Myths & Latent Print Development Techniques

CSI: Crime Scene Investigation first aired on CBS television on Friday, October 6^th 2000 (Cohan, 2008) and has been one of the top television shows on the air since then. Currently it is estimated that over 60 million people watch the CSI television show every week (Heinrick, 2006). Since the airing of the first show, America has seen a 250% increase in the amount of programs featuring forensic science as a course of study (Dutelle, 2006). While the show has made “superglue fuming” and “latent prints” household terms, the effect that the show has on the public perception on forensic science can not be ignored.  The show depicts scientific evidence being gathered, processed, and used to solve major crimes. The characters of the show use costly and high techniques to produce compelling evidence connecting a suspect to a specific crime and reconstruct the crime itself all within one hour. The show depicts that forensic evidence is gathered at every crime scene and the evidence, not the actual investigation, solves the criminal case. The show often depicts unrealistic ideas of what surfaces can be printed. 

Latent fingerprints are the most common form of fingerprint evidence and also the one that poses the most problems (Voss-De Haan, 2006). Latent fingerprints consist of a few micrograms of material transferred from the ridge skin to the surface when the fingers touch a surface. This material is a mixture of natural secretions of the body as well as contaminants from the environment (Voss-De Haan, 2006). Most latent prints are made of perspiration, which exudes from the pores of the skin in the ridges of the print (Allen, 2007). Perspiration contains about 98% water, but the mixture of different organic and inorganic compounds vary depending on an individual’s eccrine and apocrine glands (Voss-De Haan, 2006). With this high concentration of water, prints have a tendency to dry out and sometimes disappear completely (Allen, 2007). Life expectancy of a print is unknown on certain surfaces, but the life expancy can be decreased due to real world conditions such as temperature, moisture, and humidity of the surface (Sampson & Sampson, 2005).

Latent fingerprints have three main categories for eight different pattern types. The first category is the arch which comprises of about 5% of all pattern types, this category consists of the plain arch and tented arch pattern type (Coppock, 2001). The loop pattern comprises of about 65% of all pattern types, and this category type consists of the right slope loop and the left slope loop (Coppock, 2001). The third category consists of the whorl, which comprises of about 30% of all pattern types. The whorl category consists of the plain whorl, central pocket loop, double loop, and accidental whorl (Coppock, 2001). The average fingerprint can contain as many as 175 individual ridge characteristics that can distinguish the fingerprint from others (Jones, 2006). However, a print usually found at a crime scene will be a partial print, only representing 20% of a full fingerprint (Jones, 2006).

Prints can be lifted off porous and non-porous surfaces. Porous surfaces would consist of paper, cardboard, and raw wood. The best latent development techniques for porous surfaces are iodine fuming and Ninhydrin techniques (Sirchie, 2002). Iodine fuming works by a mechanism of interaction of physical absorption (Lee & Gaensslen, 2001). When iodine crystals are warmed, they produce a violent iodine vapor that is absorbed by the fingerprint secretion residues. The latent print then takes on an appearance of a yellowish brown color (Lee & Gaensslen, 2001). Ninhydrin, on the other hand is not used in a process of fuming like iodine or Cyanoacrylate (superglue). Ninhydrin is a biological stain that reacts with the amino acid of latent prints. The amino acids form a permanent chemical bond with the Ninhydrin and the latent prints will become visible (Sirchie, 2002). Ninhydrin solutions can be applied by spraying, swabbing, or dipping a surface containing a possible latent print (Lee & Gaensslen, 2001).

Non-porous surfaces include painted wood or metal, glass, plastics, and polished surfaces. The best latent print development techniques for non-porous surfaces are oxide, florescent, magnetic, metallic, or a combination of print powders (Sirchie, 2002). Fingerprint powders rely on the mechanical adherence of powder particles to the moisture and oily components of skin ridge deposits of the latent prints (Lee & Gaensslen, 2001). It should be noted that just because these methods are the reported best technique for recovery of latent prints from porous and non-porous surfaces it does not mean a latent print can be recovered every time, like on CSI.

In a CSI: Crime Scene Investigation episode called “Burked” (Mendelsohn, Zuiker, & Cannon, 2001),  Detective Brass asks a CSI technician, “Can you get a print off those balloons?” The technician replies “I can get a print off of air”. Statements like this one may make viewers believe that fingerprint evidence is always recoverable (VanLaerhoven & Anderson, 2009). Actually, fingerprints may not be available for a number of reasons. Many materials are not conducive to retaining prints and weathering of surfaces may have also removed any prints (VanLaerhoven & Anderson, 2009). Rarely people leave crisp detailed complete latent prints behind that are seen weekly on CSI. It is entirely possible to touch an item and not leave behind a latent print. When most people touch something, they only contact the surface with part of the fingerpad, and often moving their fingers creating a smudged print (VanLaerhoven & Anderson, 2009).  A smudged print can not be used for comparison and hold very little evidentiary value. For example, it is particularly difficult to retrieve a latent print from rough surfaces such as wood, cloth, skin, cardboard, or Styrofoam. Latent prints may also be partial prints, smeared, or on top of each other (Fischer, 2008). Realistically, smooth shinny surfaces are the best sources for clear, complete latent prints suitable for comparison and identification (Fischer, 2008).